Segment Routing Explicit Path

Segment routing for traffic engineering (SR-TE) uses a SR policy to steer traffic through the network. An SR-TE policy path is expressed as a list of segments that specifies the path, called a segment ID (SID) list. Each segment is an end-to-end path from the source to the destination and instructs the routers in the network to follow the specified path instead of following the shortest path calculated by the IGP. If a packet is steered into an SR-TE policy, the SID list is pushed on the packet by the head-end. The rest of the network executes the instructions embedded in the SID list. An SR-TE policy is identified as an ordered list (head-end, color, end-point):

1. Head-end – L3: Where the SR-TE policy is instantiated
2. Color – 128 and 129: A numerical value that distinguishes between two or more policies to the same node pairs (Headend – End point)
3. End-point – PE7: The destination of the SR-TE policy.

By the end of the post, we will understand how two different services originating from same PE router can have different path. The paths are created on the basis of the offered SLA.

The basic configuration is used as the same as in the earlier post of Segment Routing On Demand Next Hop for L3VPN (ODN). Below topology is used for reference purpose.

On PE7 we have created two vrf, vrf test and vrf test_without_odn. is part of vrf test and is part of vrf test_without_odn. Both are VPNv4 routes. Color 128 is set for and color 129 is set for In this post, we will be creating two explicit SR path policies and instantiate it to same end point which is PE7 but both VPNv4 service prefixes will be having different treatment. This all will be programmed on headend node which is L3.

Below is the output of node L3 beefore explicit path being implemented. The below traceroute is based on the path selected by IGP.

RP/0/0/CPU0:L3#traceroute sr-mpls From L3, prefix-sid is used to reach PE7.
0 MRU 1500 [Labels: 16007 Exp: 0]
L 1 MRU 1500 [Labels: 16007 Exp: 0] 10 ms
L 2 MRU 1500 [Labels: 16007 Exp: 0] 0 ms
L 3 MRU 1500 [Labels: implicit-null Exp: 0] 10 ms
! 4 10 ms

Under Segment Routing Traffic Engineering, we have created a explicit segment list 142567 which is having the loopback address or node address of each node. By default IGP selects L3-S1-P5-P6-PE7 path but the below explicit segment-list defines L3-S1-L4-S2-P5-P6-PE7 path. This path can be defined by using prefix-sid also. For same destination, one more explicit path is created PATH_TEST_COLOR_129 which is having only SIDs and path is different. So based on offered services, paths can be defined.

segment-list 142567
index 10 address ipv4 -> Using Loopback IP address of Nodes
index 20 address ipv4
index 30 address ipv4
index 40 address ipv4
index 50 address ipv4
index 60 address ipv4
segment-list PATH_TEST_COLOR_129
index 10 mpls label 16004 -> Using prefix-sid value of Nodes
index 20 mpls label 16007
policy 3-to-7
color 128 end-point ipv4 -> Whenever L3 receives route with end-point with color 128, it used segmen-list 142567
preference 100
explicit segment-list 142567
policy L3toL7_Color129
color 129 end-point ipv4
preference 100
explicit segment-list PATH_TEST_COLOR_129 -> Whenever L3 receives route with end-point with color 129, it used segment-list 142567


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